dt-bindings: riscv: Add xmipsexectl ISA extension description

dt-bindings: riscv: Add xmipsexectl ISA extension description

The xmipsexectl extension is described in the MIPS RV64 P8700/P8700-F
Multiprocessing System Programmer’s Guide linked at [1].

Link: https://mips.com/wp-content/uploads/2025/06/P8700_Programmers_Reference_Manual_Rev1.84_5-31-2025.pdf
Signed-off-by: Aleksa Paunovic <aleksa.paunovic@htecgroup.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20250724-p8700-pause-v5-1-a6cbbe1c3412@htecgroup.com
Signed-off-by: Paul Walmsley <pjw@kernel.org>